Note:
Complexity depends on the hash function and collision resolution scheme, but may be constant (Θ(1)) if the table is big enough or grows. Some open addressing schemes suffer from clustering more than others.

The table may be an array of buckets, to handle some numbers of collisions easily, but some provision must still be made for bucket overflow.

More information

Historical Note
"The idea of hashing appears to have been originated by H. P. Luhn, who wrote an internal IBM memorandum in January 1953" [Knuth98, 3:547, Sect. 6.4]. He continues with more than a page of history.